In a communications transceiver for a wireless communications system, a local oscillator (LO) generates a signal with a predetermined frequency to be mixed with transmit and/or receive signals. An LO design may include a voltage-controlled oscillator (VCO) coupled to a frequency divider circuit used to divide down the frequency of the VCO output. The LO output may be coupled to a mixer that mixes the VCO output signal with another signal to generate a signal having an upconverted or downconverted frequency. A VCO buffer may be provided between the VCO output and the frequency divider or mixer to isolate the VCO output from subsequent loads.
There are often stringent requirements on the allowable in-band and out-of-band phase-noise generated at the LO output. For example, in the GSM and CDMA communication systems, the out-of-band phase noise requirement can be difficult to meet under a given power budget. The far-offset phase noise of the LO output is often dominated by contributions from the VCO buffer and the frequency divider. With proper circuit design, the VCO buffer may be eliminated; however, the frequency divider may still contribute significant phase noise. Reducing the phase noise of the frequency divider is typically achieved only by consuming a great deal of power. Similarly, the provision of a VCO and a mixer as separate circuit blocks may also consume significant power.
It would be desirable to reduce power consumption by integrating the functionality of the VCO with the frequency divider or mixer, while minimizing both near-offset and far-offset phase noise at the LO output.